(1) Field of the Invention
This invention relates to a method of simultaneously removing a cured, permanent photoimagable solder mask from printed circuit boards and the metal surfaces thereon.
(2) Background of the Invention
Printed circuit boards are often mass produced in the industry today. These boards generally comprise a reinforcement of fiberglass or other suitable material coated with an epoxy or other resin to form an insulating dielectric laminate. The laminate is laminated with electrically conductive materials such as copper for forming conductor traces later on. The laminate and conductor traces are then drilled to receive components and the metal (usually copper) on the board is then selectively etched to produce conductive traces over the board. A smooth surface rich in resin is exposed when the conductive material is etched away from the outer surface of the board. This smooth resin rich surface is called the "buttercoat". Prior to the components being stuffed or placed on a board, a permanent solder mask film is applied to the surface of the board and selected surfaces of the conductive metal traces to prevent solder from adhering to areas of the conductor traces where solder is not desired or from damaging the board during later processes. The permanent solder mask may be applied to the area to be protected either by screen printing the desired image in the desired location or by applying permanent photoimagable solder mask to the entire surface of the board and selected areas of the conductive metal traces and photoprinting the desired image onto the surface then developing the image.
This invention pertains to removal of cured permanent photoimagable solder masks from the insulating dielectric laminate and conductive traces simultaneously. The permanent photoimagable solder mask may be a liquid, a paste or ink type, or a dry film. The mask is selectively exposed through a stencil called a "phototool" by passing high intensity ultraviolet light through the phototool. This exposes the permanent photoimagable solder mask except for the opaque areas in the phototool. This exposure to high intensity ultraviolet light causes the areas exposed to the light to harden and become more chemically resistant. The board is then passed through a developer and the selected areas not exposed to the light are developed away leaving the pads of the conductor traces and drilled holes exposed for later processes. The remaining mask becomes permanently secured to the board over the insulating dielectric laminate and areas of the conductive metal traces.
The board is then cured by baking it in an oven. Upon being fully cured the board is then inspected for defects in the solder mask or for misregistration. If misregistration of the phototool occurs during the solder masking process, then the exposed areas of the board will not be properly aligned and the solder or tin would not adhere to the proper areas of the conductors during later processing. If left in this condition, the board would be unusable. Virtually all the materials and most of the labor required to manufacture the board have been expended by the time the board reaches the inspection process.
The solder mask of the type to be removed are those types for example manufactured by Dynachem, a division of Morton Thiokol, Inc. sold under the registered trademark "Laminar DM" dry film solder mask and under the trademark "Dynamask KM". Although the chemical composition of the solder mask is a trade secret, the literature represents that of the Dynamask KM utilizes epoxy chemistry. The chemical composition of these dry film solder masks is a negative resist type which is permanent in nature and referred to in the industry as "permanent".
These are different from temporary photo-polymers used in the etching processes such as those of the Celeste type described in U.S. Pat. No. 3,469,982 to Celeste; No. 4,064,287 to Lipson et al; No. 4,592,787 to Johnson et al; No. 4,776,892 to Steppan; or the types such as the silicone or polyvinyl cinnamates described in U.S. Pat. Nos. 3,673,099 to Corby et al; No. 4,744,834 to Hag; No. 4,428,871 to Ward et al; or No. 4,770,713 to Ward. The masks described in the above patents are developed such that they cover only selected areas of the conductive metal. The metal is etched away exposing the insulating laminate and the remaining mask is adhering to the remaining traces only. The photoimagable films described in these patents are materials which are easily removed from the conductive metal traces by solvents and mechanical actions such as rubbing, brushing or abrading as described in the patents.
The solder mask which is permanently affixed to the board over the insulating laminate and conductive metal is described by the literature of the manufacture as resistant to most aqueous and solvent defluxing medias and other known methods of removal. It is these films that are sought to be removed from the insulating laminate and conductive metal traces simultaneously by the method described herein.
To make the board which is misregistered usable, the solder mask would have to be stripped from the surface of the board and the solder masking process repeated.
Besides misalignment or misregistration of the phototool in the exposure of the permanent photoimagable solder mask, processing defects such as blistering or other evidence of poor adhesion may occur requiring the stripping of the permanent photoimagable solder mask.
In the past, several methods to remove solder mask have been used with only varying degrees of success. One method used a strong organic solvent such as methylene chloride or other chlorinated solvents often mixed with aldehydes or ketones such as acetone. This solution used at room temperature required a relatively long soak or exposure time of over 8 hours to remove the solder mask. This led to the solution attacking the underlying surface of the laminate and exposing the reinforcement or fiberglass because the solvent would dissolve part of the buttercoat. Even the bond between the connector traces and the laminate might be weakened reducing the structural integrity of the board. Exposing the reinforcement can result in water absorption from the atmosphere which can lead to shorting of the components. Often boards which have had solder mask removed in this matter will not meet industry and military specifications because of the roughened surface and exposed reinforcing weave. Further this method requires vigorous mechanical scrubbing which tends to break down the surface and expose the connectors to mechanical damage. These solutions are inherently toxic and cause environmental problems in their disposal.
Another method used in the past, utilized a strong alkali such as sodium hydroxide or potassium hydroxide mixed with lower alcohols. This solution used at room temperature causes the coating to swell and release from the substrate. However, the alcohols are extremely flammable and produce a hazardous material which is difficult to dispose of properly. Further this method typically takes eight (8) or more hours to remove permanent photoimagable solder mask and it generally comes off in large pieces which may adhere to an adjacent board making cleanup of the board more difficult Lower alcohol mixtures used to remove solder mask may also contain amines such as ethylene diamine or ethanolamine. Amines are very toxic and interfere in the waste treatment of metal ions such as copper, etc. also found in industrial waste water.
Another method utilized in the past uses a rather low concentration of strong alkali mixed in water and mixed with a high concentration of glycol ethers such as ethylene glycol butyl ether or diethylene glycol butyl ether. Typically, the alkali concentration is less than five (5) percent by weight, the water concentration is typically less than fifteen (15) percent by weight and most of the balance of the remover is glycol ethers. The higher concentration of glycol ethers may attack the buttercoat of the laminate. This solution typically used at room temperatures generally takes eight (8) or more hours to operate. The permanent photoimagable solder mask comes off in large pieces which is inconvenient and difficult to clean up.
Both the methods described above using alkali removers work on the solder mask from the edge of the board as the solution works its way under the surface of the permanent photoimagable solder mask. In addition, the remover breaks the bond of the permanent photoimagable solder mask over the conductive traces first. Then the remover works from the edge of the board and the edges of the conductive traces inwardly Thus, the conductive traces are exposed for a longer period of time possibly weakening the bond of the conductor to the board. As the remover works inward from the conductive traces and the edge of the board, some areas of the buttercoat are exposed to the remover. The exposed areas of the buttercoat are attacked as they are exposed and these portions of the buttercoat may be damaged. The alkalis tend to attack and tarnish the copper of the conductor traces or subject it to rapid oxidation once it is removed from the bath.
All of the above methods permit the permanent photoimagable solder mask to come off in large pieces which then float in the remover and often times adhere to an adjacent board making cleanup of the board more difficult.
None of these methods work well on solvent processable permanent photoimagable solder mask.
Another method sometimes used is known as the abrasive blast method which is similar to sand blasting wherein abrasive particles are sprayed under high pressure onto the surface of the board to remove the solder mask. The blasting of the solder mask results in a loss of or damage to the buttercoat and abrasion to the conductors.